Certain bodily processes take two or three fewer steps to perform in a tunic than in pants. So why all the pants?

According to University of Connecticut evolutionary biologist Peter Turchin, pants owe their several thousand years of worldwide fashionableness to horses — or, more precisely, to the extreme awkwardness of riding a horse in a robe. "Historically there is a very strong correlation between horse-riding and pants," he wrote in a recent article for the Social Evolution Forum.

Turchin points to examples of this correlation ranging from Japan, where the traditional dress is the kimono but where samurais wore baggy trousers, to North America, where Plains Indians donned kilts until Europeans brought horses to the continent. Roman soldiers mounted steeds (and adopted pants) in the first century A.D. after getting trounced repeatedly by Hannibal and his trouser-clad cavalrymen.

A few centuries earlier in pre-unified China, switching from robes to pants became a matter of state survival in the face of invasion by pants-wearing nomadic horsemen from Central Asia. Soldiers in many of the Chinese states greatly resisted this "barbarian" legwear, and either galloped uncomfortably in robes or left off horses altogether. It cost them everything. "Pants won in China by the process of cultural group selection," Turchin wrote. "Those states that did not adopt cavalry (and pants), or adopted them too slowly, lost to the states that did so early." [ Top 10 Inventions that Changed the World ]

Pants-wearing became an everyday affair in Europe during the eighth century, after the fall of the Roman Empire, "when the continent fell under the rule of warriors who fought from horseback — the knights," Turchin explained. "So wearing pants became associated with high-status men and gradually spread to other males."

The connection between pants and horse-riding also explains why women stuck to skirts until recently — except, of course, for the female Amazon warriors. They wore pants.